Titanium nitride (TiN) films have several applications, including use as a barrier layer or a local interconnect in semiconductor devices. As an example, a titanium nitride film in a semiconductor device is often formed between a layer of aluminum or an aluminum alloy and a silicon substrate to prevent a problem known as "spiking." Spiking occurs in a semiconductor device at elevated temperatures (e.g. in excess of 400.degree. C.) in portions of the device where aluminum is in contact with silicon or has the ability to come in contact with silicon by diffusion through other layers. The solubility of silicon in aluminum at elevated temperatures is quite high, such that aluminum will take silicon into solution. As a result of the aluminum "consuming" the silicon, voids or pits can be formed in the silicon substrate. These pits are very regularly shaped and often look like upside-down pyramids formed in the silicon substrate because the silicon substrate is usually monocrystalline silicon and therefore has a specific crystallographic orientation. If pitting happens to occur in portions of the silicon substrate in which junctions have been formed, electrical characteristics of the junctions are significantly and undesirably altered, often leading to junction leakage. Because spiking can result in junction leakage, the phenomenon is also known as "junction spiking." By providing a barrier layer between the silicon substrate and aluminum, the problem of spiking can be reduced. The presence of a barrier layer serves to block the diffusion of silicon and aluminum such that the two materials have a reduced chance of meeting and going into solution.
Titanium nitride is one of a few materials which is commonly used as a barrier layer in semiconductor devices. A widely practiced approach to forming a TiN barrier between silicon and aluminum is to first deposit a layer of titanium onto a silicon substrate. The silicon substrate with the titanium layer is then annealed by placing the substrate in a reaction chamber and heating the substrate to, for example, a temperature near 600.degree. C. or higher. An ammonia gas (NH.sub.3) at room temperature is pumped into the reaction chamber and reacts with the layer of titanium to form a thin layer of titanium nitride (TiN) on the surface of the titanium layer. The reaction occurs in accordance with the equation 2Ti+4NH.sub.3 .revreaction.2TiN+N.sub.2 +6H.sub.2. The thickness of the resulting TiN layer is determined by the amount of time the titanium layer is exposed to the ammonia, as well as the temperature of the substrate and other processing variables. Many semiconductor manufacturers employ a rapid thermal anneal (RTA), on the order of ten seconds to two minutes, to form a TiN barrier layer. A typical TiN barrier layer thickness formed by the above mentioned RTA process may be on the order of 50 to 400 .ANG. (5-40 nm).
However, even with a barrier layer, spiking can occur upon exposure of a semiconductor device to elevated temperatures, especially if the exposure is for an extended period of time. Silicon and aluminum atoms become more mobile as temperature increases, allowing silicon to diffuse into an overlying titanium layer. At the same time, aluminum diffuses through a thin TiN barrier layer formed on the titanium layer and subsequently into the titanium layer. Eventually, the aluminum and silicon diffuse to the extent that aluminum atoms and silicon atoms are in close enough proximity to form a solution. As a result, silicon is consumed by aluminum, leaving voids or pits. Therefore, the use of barrier layers does not completely prevent junction spiking at elevated temperatures. Instead, the presence of a barrier layer more or less serves to delay the problem. In using barrier layers in semiconductor devices, manufacturers hope to delay spiking sufficiently to avoid the formation of pits at temperatures which a semiconductor device might experience. However, existing barrier layers and processes for forming these barrier layers are often inadequate in substantially preventing silicon consumption by aluminum.